linux-kernel - Re: [PATCH 01/11] sysfs: Support for preventing unmounts.

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Date:	Thu, 19 Jun 2008 10:54:49 +0200
From:	Benjamin Thery <benjamin.thery@...l.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <htejun@...il.com>, linux-kernel@...r.kernel.org,
	Al Viro <viro@....linux.org.uk>,
	Linux Containers <containers@...ts.osdl.org>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Daniel Lezcano <dlezcano@...ibm.com>
Subject: Re: [PATCH 01/11] sysfs: Support for preventing unmounts.

Eric W. Biederman wrote:
> Dave Hansen <dave@...ux.vnet.ibm.com> writes:
> 
>> On Wed, 2008-06-18 at 19:07 +0200, Benjamin Thery wrote:
>>> To support mounting multiple instances of sysfs occassionally I
>>> need to walk through all of the currently present sysfs super blocks.
>> I know you may have addressed this before, but I forgot and it didn't
>> make it into the changelogs.
>>
>> Why are you doing this again?  It seems like an awfully blunt
>> instrument.  
> 
> So the fundamentals.
> - The data in sysfs fundamentally changes behind the back of the
>   VFS and we need to keep the VFS in sync.  Essentially this is the
>   distributed filesystem problem.
> 
> - In particular for sysfs_rename and sysfs_move_dir we need to support finding
>   the dcache entries and calling d_move.  So that the dcache does not
>   get into an inconsistent state.  Timeouts and invalidates like NFS
>   uses are to be  avoided if at all possible.
> 
> - Coming through the vfs we are guaranteed that the filesystem will
>   not be unmounted while we have a reference on a dentry, and with
>   multiple mounts we do not get that guarantee.  Therefore to get that
>   guarantee for all of the superblocks we need the blunt instrument.
> 
> - Since mount/unmount are rare blocking them is no big deal.
> 
> I believe any distributed filesystem that is together enough to tell
> us about renames (so we can update the dcache) instead of doing the
> NFS timeout will need the ability to block mount/unmount while it is
> executing d_move.
> 
> Currently sysfs does not need to block mounts only because we perform
> an internal mount and then never unmount sysfs.

Thanks Eric for detailing this.
I think you explained it in much better way than I could do.
You're the author of the patch after all ;-)

  Benjamin

> 
> Eric
> 
> 


-- 
B e n j a m i n   T h e r y  - BULL/DT/Open Software R&D

    http://www.bull.com
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